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TEM images of (a) UCNPs and (b) UCNPs@SiO2. (c) X-ray diffraction pattern and reference diffraction peaks corresponding to hexagonal NaGdF4 (JCPDS file 27-0699). (d) Upconversion emission spectra (λexc 980 nm) at different temperatures (293 and 323 K) of UCNPs@SiO2.
